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For example, I have a table called table1 that has two colums:
fld1, fld2... where fld2 is a NULL colum. I need to write a trigger that determines before an insert or update it has to check column fld2 and throw an error if trying to insert or update a duplicate value:

CREATE OR REPLACE TRIGGER SomeJob_TRIG
BEFORE INSERT OR UPDATE ON table1
FOR EACH ROW